LOWET? i;iO liRAVO. 35 



The upper part of this belt commences in the vicinity of Las Moras, and terminates some few 

 miles above Laredo, a distance of about 200 miles, whilst the lower part begins where the former 

 ends, and reaches as far as the vicinity of Reynosa, showing a width of about 340 miles. Both 

 of these parts are distinctly characterized by strata of green sand, (chloritic chalk,) which 

 change, according to the amount of oxyde of iron they contain, into variously tinted sandstone 

 shoals. The solidity of the strata varies very much. They are sometimes formed into very 

 solid rocks, well suited for mechanical or architectural operations ; again, they consist of loose 

 and coarsely-grained sandstone slate, which rapidly crumbles on exposure to the air. All these 

 green sand strata are frequently intersected with layers of debris of analogous character. 



In several places where these green sand strata were disintegrating, and being carried off by 

 the action of the waters, there was observed a white, salty efflorescence, which may possibly be 

 "ammonia." The "Rocky walls" near the mouth of the Arroyo Castauo, which is about 40 

 miles below Eagle Pass, and near the Presidio de San Juan el Bautista, are remarkable for this 

 efflorescence, as also some terraces below this point. The frequent occurrence of a certain cheno- 

 podium, containing a large amount of this salt, and often covering exclusively wide tracts of 

 sandy bottom-land along the Rio Bravo, may prove more conclusively the peculiar elements of the 

 green sandstone. 



The green sand, particularly in the upper belt, is often and variously intersected by strata of 

 different nature, though certainly closely allied with the same system. 



Strata of sandy or argillaceous marls, or blue or grayish clay, all profusely impregnated with 

 oxide of iron, and even layers of corresponding debris, often intersect the green sand strata. 



The general characteristic of this belt and its subdivisions is the strict horizontality of its 

 strata throughout. It is only here and there that some slight local disturbance has taken place, 

 as, for instance, near Laredo, and again some 40 or 50 miles above, where a dip of about 8° 

 W.S.E. and E. is exposed. 



The following peculiarities may serve to characterize the two subdivisions of the green sand belt : 



LIGNITE COAL. 



From Las Moras to the vicinity of Arroyo Sombreretillo, which is about 10 miles above 

 Laredo, lignite coal occurs quite frequently. None came iinder our observation below this point ; 

 outcrops of it, however, are said to be found in the neighborhood of Roma, some 10 miles above 

 the mouth of the Rio San Juan. 



Though there is not much doubt of the existence of lignite below the Arroyo Sombreretillo, 

 our observations have led us to the conclusion that it is more sparsely distributed. 



These lignites vary both in appearance and quality; sometimes they are found to be scaly 

 or slaty, and of a dull earthy fracture, sometimes resinous and sharply edged. Prints, and 

 even remains, of plants, preserved in these coals, indicate vegetable forms of the higher 

 orders, as gramincfs, (perhaps reed and cane,) and even parts of dicotyledonous trees, such as 

 willow or ash. Other specimens of coal from below appear more amorphous ; but it contains so 

 much bitumen as to be of no use in the blacksmith's forge, where it runs together and becomes 

 baked into a solid mass. 



The localities remarkable for the most considerable deposits of lignite coal are the following : 



On both sides of the mouth of Elm creek, near Eagle Pass, particularly on the north bank of 

 this water-course, where layers from 3 to 4 feet thick are exposed. On the south bank of this 



